Background calibration of sampler offsets in analog to digital converters

ABSTRACT

A method for background calibration of sampler offsets in an Analog to Digital Converter (ADC), according to which one of the samplers of the ADC is established as a reference sampler, whose threshold and timing offsets will be the criterion for adjusting threshold offsets and timing offsets of all other samplers. Then each of the other samplers of the ADC, one at a time, is calibrated by selecting an uncalibrated sampler and establishing it as the current Sampler Under Calibration (SUC); disregarding contribution of the SUC to the output of the ADC; adjusting the threshold of the SUC to be identical to the threshold of the reference sampler; performing one-bit cross-correlation between the reference sampler and the SUC; establishing an error surface representing the threshold offset and timing offset of the SUC with respect to the reference sampler; adjusting the threshold and the timing of the SUC to be equal to the threshold and timing of the reference sampler; restoring level of the SUC to its original threshold with respect to the overall ADC and restoring contribution of the SUC to the output of the ADC.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/236,194, filed Oct. 2, 2015, the disclosure of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of Analog to DigitalConverters (ADCs). More particularly, the invention relates to a methodfor calibrating samplers of ADCs.

BACKGROUND OF THE INVENTION

A flash ADC resolves an analog signal level to a digital number bycomparing the analog signal at an instant in time to a set ofthresholds. This comparison is performed by a set of analog samplers,which are also called clocked comparators.

The clocked comparator introduces an offset to the desired threshold, inaddition to other imperfections. Threshold errors in an ADC introducedistortion in the resolved digital signal and therefore degrade theADC's signal to noise and distortion ratio (SNDR). In addition, clockedcomparators may include timing errors, which contribute to the overallnoise and distortion of the ADC.

Sampler offsets may vary in a number of manners, such as:

-   -   differences between samplers;    -   differences between dies; and    -   differences between temperatures, processes, power supply        voltages, which appear over time.

A method for calibrating a sampler by introducing special test signals,known as “foreground calibration”, introduces a time when the ADC is notresolving the true input signal, and this is not acceptable for someapplications.

Another existing method for calibrating samplers is by using specialcircuitry, such as Digital to Analog Converters. This solution, however,is expensive in terms of die area and power dissipation.

An alternative method is a onetime calibration (e.g., at power-up) wherethe calibration scheme does not interfere with the transmission schemeonce the latter has begun. This method however is not optimal, since thecalibration will degrade over time, which entails SNDR degradation overtime, when the errors themselves change over time, due to the fact thattemperature or other conditions change.

It would be advantageous to be able to remove ADC sampler offsetswithout interfering normal data transmission, while maintainingcalibration throughout the entire transmission scheme, using aneconomical system.

It is therefore an object of the present invention to provide a methodfor calibrating ADC samplers and removing ADC sampler offsets, withoutinterfering data transmission, while maintaining calibration through theentire transmission scheme using an economical system.

Other objects and advantages of this invention will become apparent asthe description proceeds.

SUMMARY OF THE INVENTION

The present invention is directed to a method for background calibrationof sampler offsets in an Analog to Digital Converter (ADC), comprisingthe following steps:

-   -   a. establishing one of the samplers of the ADC as a reference        sampler, whose threshold and timing offsets will be the        criterion for adjusting threshold offsets and timing offsets of        all other samplers; and    -   b. calibrating each of the other samplers of the ADC, one at a        time, by:        -   b.1) selecting an uncalibrated sampler and establishing it            as the current Sampler Under Calibration (SUC);        -   b.2) disregarding contribution of the SUC to the output of            the ADC;        -   b.3) adjusting the threshold of the SUC to be identical to            the threshold of the reference sampler;        -   b.4) performing one-bit cross-correlation between the            reference sampler and the SUC;        -   b.5) establishing an error surface representing the            threshold offset and timing offset of the SUC with respect            to the reference sampler;        -   b.6) adjusting the threshold and the timing of the SUC to be            equal to the threshold and timing of the reference sampler;        -   b.7) restoring level of the SUC to its original threshold            with respect to the overall ADC;        -   b.8) restoring contribution of the SUC to the output of the            ADC.

The reference sampler may be a sampler whose threshold is at the middlelevel of the ADC.

The step of disregarding the contribution of the SUC to the ADC's outputmay comprise disconnecting the SUC from the thermometric output code ofthe ADC.

The step of disregarding the contribution of the SUC to the ADC's outputmay comprise digitally removing the contribution from within thethermometric output code using bubble code removal.

The step of adjusting the threshold and timing of the SUC to be equal tothose of the reference sampler may comprise following the gradientdescent of the error surface, until the timing and threshold of the SUCequal those of the reference sampler, wherein the outputs of the SUC andthe reference sampler are continuously sampled, and thecross-correlation between the two is continuously calculated.

In one aspect, the step of adjusting the threshold and timing of the SUCto be equal to those of the reference sampler comprises:

-   -   a. adjusting the threshold of the SUC to be equal to that of the        reference sampler; and    -   b. adjusting the timing of the SUC to be equal to that of the        reference sampler,

wherein the outputs of the SUC and the reference sampler arecontinuously sampled, and the cross-correlation between the two iscontinuously calculated.

In one aspect, the step of adjusting the threshold and timing of the SUCto be equal to those of the reference sampler comprises:

-   -   a. establishing a grid of arbitrarily selected points on the        error surface;    -   b. calculating the cross-correlation between each point and the        reference sampler; and    -   c. adjusting the threshold and timing of the SUC to be equal to        those of the point that has been found to have the highest        cross-correlation with the reference sampler.

In one aspect, each sub-ADC of the TIADC is individually calibratedaccording to the steps above.

The present invention is also directed to an Analog to Digital Converter(ADC) having background calibration capability of sampler offsets, inwhich:

-   -   a. one of the samplers is established a reference sampler, whose        threshold and timing offsets will be a criterion for adjusting        threshold offsets and timing offsets of all other samplers; and    -   b. each of the other samplers are calibrated, one at a time, by:        -   b.1) selecting an uncalibrated sampler and establishing it            as the current Sampler Under Calibration (SUC);        -   b.2) disregarding contribution of the SUC to the output of            the ADC;        -   b.3) adjusting the threshold of the SUC to be identical to            the threshold of the reference sampler;        -   b.4) performing one-bit cross-correlation between the            reference sampler and the SUC;        -   b.5) establishing an error surface representing the            threshold offset and timing offset of the SUC with respect            to the reference sampler;        -   b.6) adjusting the threshold and the timing of the SUC to be            equal to the threshold and timing of the reference sampler;        -   b.7) restoring level of the SUC to its original threshold            with respect to the overall ADC;        -   b.8) restoring contribution of the SUC to the output of the            ADC.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1a schematically shows an ADC with 11-samplers, which is to becalibrated, according to an embodiment of the present invention.

FIG. 1b schematically shows the establishment of a reference sampler forcalibration, according to an embodiment of the present invention.

FIG. 1c schematically shows the establishment of a sampler undercalibration, according to an embodiment of the present invention.

FIG. 1d schematically shows a sampler under calibration whose thresholdhas been adjusted to be equal to that of a reference sampler, accordingto an embodiment of the present invention.

FIG. 2 schematically shows an error surface representing the thresholdand timing offsets of a sampler under calibration with respect to areference sampler, according to an embodiment of the present invention.

FIGS. 3a-3c schematically show methods of correcting threshold andtiming offsets of a sampler under calibration, according to embodimentsof the present invention.

FIG. 4 schematically shows a calibrated sampler returned to its originalthreshold with respect to the overall ADC, according to embodiments ofthe present invention.

FIG. 5 schematically shows an ADC with 11 samplers which have allundergone calibration, according to embodiments of the presentinvention.

FIG. 6 shows a flowchart which describes the process of calibratingtiming and threshold offsets of a sampler under calibration, accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention proposes a method for background calibration ofADC samplers and removing ADC sampler offsets.

FIGS. 1a-1d and FIG. 2 schematically illustrate a process of evaluatingan error surface representing the threshold and timing offsets of asampler, according to an embodiment of the invention. In FIGS. 1a-1dnumeric 100 is a representation of 11 samplers in an uncalibrated ADC.The horizontal axis is the time axis, and the vertical axis is thevoltage axis, i.e. threshold axis. Samplers are represented in FIGS.1a-1d as dots. Timing offsets are schematically shown as verticalnon-alignment of dots, and threshold offsets are schematically shown asdeviation of dots from the vertical axis.

FIG. 1a schematically illustrates an ADC with 11-samplers, which is tobe calibrated. According to the invention, one of the 11 samplers of theADC is established as a reference sampler, numeric 101 in FIG. 1b .According to an embodiment of the invention, the sampler whose thresholdis at the middle level of the ADC is determined as the referencesampler.

All other 10 samplers in the ADC will be calibrated with respect toreference sampler 101. After establishing the reference sampler, each ofthe other samplers undergoes the calibration process described below,one at a time.

According to an embodiment of the invention, a first sampler which is tobe calibrated, numeric 102 in FIG. 1c , is disconnected from thethermometric output code of the ADC. According to another embodiment ofthe invention sampler 102 continues to contribute to the thermometricoutput, although the contribution of 102 will be removed digitally, e.g.by bubble code removal.

Next, as shown in FIG. 1d , the level of sampler 102 is adjusted to beidentical to that of reference sampler 101. Once the level is adjusted,one-bit cross-correlation is performed, wherein a series of samples istaken from the outputs of samplers 101 and 102. The samples from 101 arecompared to the samples from 102, and an error surface is received,which represents the cross-correlation result.

FIG. 2 schematically illustrates an error surface 203 representing thethreshold and timing offsets of a sampler under calibration (hereinafterSUC), 102, with respect to a reference sampler, 101. The outputs of thereference sampler and the SUC are shown in FIG. 2 as numerical 201 and202, respectively. The horizontal axis of surface 203 represents thetiming of the samples, while the horizontal axis of surface 203represents the thresholds of the samples.

Once the error surface has been evaluated, the threshold and timingoffsets of the SUC are adjusted to equal zero, with respect to theoffsets of the reference sampler.

FIG. 3a schematically illustrates a method of correcting threshold andtiming offsets of a SUC according to an embodiment of the invention. InFIG. 3a the threshold and timing offsets of a SUC are concurrentlyadjusted by following the gradient descent 301 of the error surface 203,until the timing and threshold of the SUC equal those of the referencesampler. The outputs of the SUC and the reference sampler, i.e. 202 and201, are continuously sampled during this adjustment, in order tocontinuously determine the cross-correlation between the two, and tofinally decide that they are equal.

FIG. 3b schematically illustrates another method of correcting thresholdand timing offsets of a SUC according another embodiment of theinvention. In FIG. 3b the offsets of a SUC are corrected one at a time,i.e. at first the threshold offset is corrected by adjusting thethreshold of the SUC until it equals that of the reference sampler.Next, the timing of the SUC is corrected by adjusting it until it isequal to that of the reference sampler. The outputs of the SUC and thereference sampler, i.e. 202 and 201, are continuously sampled during theadjustment, in order to continuously determine the cross-correlationbetween the two, and to finally decide that they are equal.

FIG. 3c schematically illustrates yet another method of correctingthreshold and timing offsets of a SUC according yet another embodimentof the invention. In FIG. 3c the offsets of a SUC are corrected byestablishing a grid 302 of arbitrarily selected points on the errorsurface 203, calculating the cross-correlation of each point with theoutput 201 of the reference sampler, and adjusting the offsets of theSUC to be equal to those of the point 303 which has been found to havethe highest cross-correlation with the output 201 of the referencesampler.

After adjusting the threshold and timing offsets of a SUC, the level ofthe SUC is restored, and is returned to its original threshold withrespect to the overall ADC, and is now re-connected to the thermometricoutput code of the ADC. FIG. 4 schematically illustrates the nowcalibrated sampler 102 returned to its original threshold with respectto the overall ADC 100. It can be seen that the timing and thresholdoffsets of sampler 102 are equal to those of reference sampler 101.

Next, each of the other nine un-calibrated samplers is calibratedaccording to the process described above. FIG. 5 schematicallyillustrates an ADC 500 with 11 samplers which have all undergonecalibration.

FIG. 6 is a flowchart which describes the process of calibrating timingand threshold offsets of a SUC of an ADC, according to an embodiment ofthe invention. At the first step 601, a reference sampler is establishedas a criterion for other samplers. The reference sampler is preferablyone of the existing samplers of the ADC, thereby avoiding the need touse a dedicated sampler for this calibration process.

The other samplers will be calibrated with respect to this sampler. Atthe next step 602, the SUC is disconnected from the thermometric outputcode of the ADC. At the next step 603, the level of the SUC is adjustedto be identical to that of the reference sampler. At the next step 604,one-bit cross-correlation is performed, and an error surface isestablished, representing the threshold and timing offsets with respectto the reference sampler. At the next step 605, the threshold and timingoffsets of the SUC are corrected to equal the offsets of the referencesampler, according to one of the methods described above. At the nextand final step 606, the SUC is restored and returned to its originallevel with respect to the overall ADC, and is re-connected to thethermometric output code of the ADC. The process has now ended, and theSUC is calibrated.

According to an embodiment of the invention, an ADC is part of a TimeInterleaved ADC (TIADC), where a plurality of low frequency sub-ADC'sare utilized in order to reach a high frequency ADC. According to thisembodiment, a TIADC can be calibrated using the calibration process, byindividually carrying out the process for each sampler of each ADC.

As various embodiments have been described and illustrated, it should beunderstood that variations will be apparent to one skilled in the artwithout departing from the principles herein. Accordingly, the inventionis not to be limited to the specific embodiments described andillustrated in the drawings.

The invention claimed is:
 1. A method for background calibration ofsampler offsets in an Analog to Digital Converter (ADC), comprising: a.establishing one of the samplers of said ADC as a reference sampler,whose threshold and timing offsets will be the criterion for adjustingthreshold offsets and timing offsets of all other samplers; and b.calibrating each of the other samplers of the ADC, one at a time, by:b.1) selecting an uncalibrated sampler and establishing it as thecurrent Sampler Under Calibration (SUC); b.2) disregarding contributionof said SUC to the output of the ADC; b.3) adjusting the threshold ofsaid SUC to be identical to the threshold of said reference sampler;b.4) performing one-bit cross-correlation between said reference samplerand said SUC; b.5) establishing an error surface representing thethreshold offset and timing offset of said SUC with respect to saidreference sampler; b.6) adjusting the threshold and the timing of saidSUC to be equal to the threshold and timing of said reference sampler;b.7) restoring level of said SUC to its original threshold with respectto the overall ADC; b.8) restoring contribution of said SUC to theoutput of the ADC.
 2. The method of claim 1, wherein the referencesampler is a sampler whose threshold is at the middle level of the ADC.3. The method of claim 1, wherein the step of disregarding thecontribution of the SUC to the ADC's output comprises disconnecting saidSUC from the thermometric output code of said ADC.
 4. The method ofclaim 1, wherein the step of disregarding the contribution of the SUC tothe ADC's output comprises digitally removing said contribution fromwithin the thermometric output code using bubble code removal.
 5. Themethod of claim 1, wherein the step of adjusting the threshold andtiming of the SUC to be equal to those of the reference samplercomprises following the gradient descent of the error surface, until thetiming and threshold of said SUC equal those of said reference sampler,wherein the outputs of said SUC and said reference sampler arecontinuously sampled, and the cross-correlation between the two iscontinuously calculated.
 6. The method of claim 1, wherein the step ofadjusting the threshold and timing of the SUC to be equal to those ofthe reference sampler comprises: a. adjusting the threshold of said SUCto be equal to that of said reference sampler; and b. adjusting thetiming of said SUC to be equal to that of said reference sampler,wherein the outputs of said SUC and said reference sampler arecontinuously sampled, and the cross-correlation between the two iscontinuously calculated.
 7. The method of claim 1, wherein the step ofadjusting the threshold and timing of the SUC to be equal to those ofthe reference sampler comprises: a. establishing a grid of arbitrarilyselected points on the error surface; b. calculating thecross-correlation between each point and said reference sampler; and c.adjusting the threshold and timing of said SUC to be equal to those ofthe point that has been found to have the highest cross-correlation withsaid reference sampler.
 8. A method for background calibration of TimeInterleaved ADCs (TIADC), comprising individually calibrating eachsub-ADC that comprise said TIADC according to the method of claim
 1. 9.An Analog to Digital Converter (ADC) having background calibrationcapability of sampler offsets, in which: a. one of the samplers isestablished a reference sampler, whose threshold and timing offsets willbe a criterion for adjusting threshold offsets and timing offsets of allother samplers; and b. each of the other samplers are calibrated, one ata time, by: b.1) selecting an uncalibrated sampler and establishing itas the current Sampler Under Calibration (SUC); b.2) disregardingcontribution of said SUC to the output of the ADC; b.3) adjusting thethreshold of said SUC to be identical to the threshold of said referencesampler; b.4) performing one-bit cross-correlation between saidreference sampler and said SUC; b.5) establishing an error surfacerepresenting the threshold offset and timing offset of said SUC withrespect to said reference sampler; b.6) adjusting the threshold and thetiming of said SUC to be equal to the threshold and timing of saidreference sampler; b.7) restoring level of said SUC to its originalthreshold with respect to the overall ADC; b.8) restoring contributionof said SUC to the output of the ADC.